Plant Diagnostics – Part 1

You spent the spring planning the perfect garden and everything looked beautiful, but now some of your plants look slightly “off”. You can tell something doesn’t seem right with a portion of your garden, but how do you know exactly what is wrong and how to fix it? Sound familiar? I know this sometimes happens to me. Countless factors influence agriculture, so when one thing goes wrong, we can see the impact on our plants but have no clue about the cause.

Plant diseases are any condition in a plant caused by living or nonliving agents that interfere with its normal growth and development. Plant diagnostics is an often-challenging process that is then used to come up with the best possible explanation for why our plants are sick and which disease is causing problems. For a disease to develop there must be a susceptible host, a pathogen capable of causing a disease, and a favorable environment for development. When all three of these things are present, the disease can spread and cause damage to our plants. In diagnostics, there are two main terms to understand: symptom and sign. Symptoms are a change in plant growth or appearance that may indicate or describe a plant health problem such as yellowing, wilting, galls, or blight. Signs are evidence of the damaging factor and the actual cause of the problem that allows you to conclusively diagnose a plant health problem. Examples of signs could include egg masses, larvae, mycelia, rust, powdery mildew, and pustules. The location of damage, type of damage, symptoms, and signs are all important clues for determining the cause of your plant concern. 

You should start the diagnosis process using a systematic approach. Consider an assortment of questions and factors while looking for patterns:

What is the plant type?
Any recent extreme weather?
How long has the plant been in that location?
What has been done to or near the plant?
Is the symptom or sign on multiple plants?
Are the diseased plants all in the same place or spread across locations?
Where on the plant is the damage? Localized or evenly throughout?
Is the problem spreading gradually or rapidly?

There are many more questions to be asked, but ultimately you should be observant and pay attention to what your plants are saying and showing you. 

Plant pathogens cause most living plant diseases and include fungi, bacteria, viruses, and nematodes. Fungi spread through the penetration of plant tissues or openings on plant surfaces. This means that they generally require free water on plant surfaces to cause an infection and are typically more common after periods of wet weather or when overhead irrigation is used. Fungal pathogens can often cause round leaf spots, stem rot with a dry/papery texture, concentric rings, discoloration, wilt, or fruiting structures on the affected tissue. Bacterial diseases can only invade your plants through wounds, often wounds made by insects. When a bacterium is present in your plants it can take the form of swollen gall areas, irregularly shaped leaf spots, wilting followed by yellowing and death, or a wet rot. Viruses also require a plant wound for infection and primarily spread through insects. When there is a viral infection in your plant you may observe yellowing or mottling from the inhibition of chlorophyll formation, stunting, distortion, or dieback on a portion of the plant. Luckily, viruses are usually debilitating rather than deadly. The larvae and adult forms of nematodes are most damaging to plants and infect a crop through direct penetration in the tip of the root. Confusingly, nematodes cause disease-like symptoms such as the shortening of internodes, damage to the root system, wilting, and stunted growth. The last biotic group that could be damaging your plants is insects which will either be chewing or sucking types. Chewing insects can cause ragged/chewed or missing leaves, rolled leaves, tunnels, holes in plant stems, or premature yellowing while sucking plants inject toxins into plants after drinking the juices which can cause leaf spotting or stippling, curling, or puckering, and often poisoning of entire plants. 

Several other nonliving factors such as mechanical, physical, light, heat, drought, waterlogging, chemical burn, and nutrient deficiencies can cause damage. These can typically be identified when thinking critically about environmental factors. Nutrient deficiencies, however, are important to recognize and often appear as yellowing, stunting, or death of either older plant leaves or new growth depending on the missing nutrient. 

You can always send plant samples to a diagnostic laboratory or consult an expert to confirm your suspicions. Remember: don’t make the symptoms fit the diagnosis, but instead make the diagnosis fit the symptoms.